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NMSU demonstrates economical way to prevent curly top virus devastation of tomato crop

LOS LUNAS, N.M. - Every year gardeners worry about losing their tomato crop to curly top virus. Those raising produce for farmers' markets wonder if they will be able to provide the much-sought-after vegetable.

NMSU staff at the Agricultural Science Center at Los Lunas work with row cover material during a study of an economical way to decrease the devastation of tomato crops caused by curly top virus. The material, placed over the plants when they are transplanted, blocks leafhoppers from transmitting the virus to the plants. (NMSU photo courtesy of Ron Walser)

Ron Walser, urban small-farm specialist at New Mexico State University's Agricultural Science Center at Los Lunas, has conducted a two-year study that demonstrates an economical way to ensure having tomatoes.

"The results were so dramatic both years that growers who have seen the study are definitely going to use it in the future. It gives them confidence that they are going to have a tomato crop," Walser said of using row cover material to cover tomato plants and block the infestation of curly top virus.

Curly top virus of tomatoes, or beet curly top virus, is transmitted by the beet leafhopper's saliva. To prevent the insect from spreading the virus when it eats the tomato plants, growers have to either eliminate the insects or prevent them from reaching the plants.

"To kill the leafhoppers with chemicals, you would have to spray daily, but the virus could still be transmitted if the insect is able to attack the plant before the insect dies," said Walser. "So this method has not been effective in preventing the curly top virus from infecting plants."

Instead of chemical spraying, a second traditional method to control the spread of curly top is weed control.

"Leafhoppers propagate in wild mustard plants during the winter and early spring. About the time those plants are drying up in May, tomatoes are being transplanted, which gives the insect a new environment to thrive in. One way to reduce the hopper's movement into the garden is to remove the weeds from around the area, but leafhoppers can be carried by the wind from a distance, and they still find the tomato plants," Walser said.

Some growers have discovered that using shade fabric helped prevent the plants from being infected as badly as plants grown in the open. Walser wanted to determine how economical it is to use a similar technique.

"We tried two ways to protect the plants. One technique was to whitewash the plants with Kaolin Clay, using a product called Surround, so the insect wouldn't want to get on the plant," he said. "But we got minimal results. In 2008 when the infestation of leafhoppers was heavy, the Kaolin Clay had some effect, but it had little or no effect when the infestation was lighter in 2009."

The second technique was to cover the tomato plant row with an ultra-light-weight, spundbound polypropylene fabric. This method proved to have the best results. The fabric --Agribon insect barrier and Agribon floating row cover -- did not interfere with the plants' growth. Sunlight, air and water were able to pass through the material, but it prevented leafhoppers from reaching the plants, and the plants thrived.

"We covered the plants immediately after transplanting them in May. It took three people an hour to cover the 100-foot row and tack down the edges to prevent the fabric from blowing," Walser said. "Since tomatoes are wind pollinated, we were concerned that the fabric would block the necessary breeze, or that it would be too hot under the fabric for the plants to grow; however, neither proved to be a problem."

The plants were protected from curly top virus until the fabric was removed in July for easier access to the plants when it was time to harvest the tomatoes. "By July, the plants were growing beyond the three-foot cages that supported the fabric and pushing though the fabric," Walser said.

The only difference in the produce yield between 2008 and 2009 came when the heavier infestation of leafhoppers in 2008 caused 50 percent of the plants that had been covered to be lost after the fabric was removed. "But during that same year, we lost 100 percent of the plants that were not covered," Walser said.

The 2008 yield was also 50 percent of what was produced in 2009, when no covered plants contracted curly top virus.

"The economics of using the shade cloth comes with the quantity of the yield," Walser said. "In 2008 we had 20 pounds of tomatoes per plant in the covered treatment, compared to four pounds per plant using the Kaolin Clay, and less than half a pound, 0.46 pounds, per plant on those raised in the open. The uncovered crop would be considered a complete loss economically."

In 2009 the covered treatment yield per plant was 40.4 pounds, while the Kaolin Clay whitewashed plants produced 19.4 pounds per plant and the uncovered, untreated plants had a 19.9 pound-per-plant yield.

"Tomatoes are one of the major produce at farmers' markets. A grower can generally get $2 or more per pound at the markets. So we made $40 per plant in 2008 and $80 per plant in 2009. Tomatoes are planted two feet apart, so a 100-foot plot, like our test plot, would contain 50 plants. The investment to cover the plants is approximately $100 for 250 feet, which includes the cost of the fabric and u-shaped anchor pins. Add to that the cost of the tomato cage, which growers should have no matter what system they use," Walser said of the portion of the study in which the tomatoes were sold at the local markets.

During the study, the Agribon fabric-- both the insect barrier and floating row cover -- cost $45 for 250-feet of coverage, with an additional $55 for u-shaped anchoring pins.

"So we basically made up our investment with one or two plants," he said. "But the most important thing is that we produced tomatoes when otherwise we might not have, because of the virus."